This invention relates to a method of preparing starch esters having an intermediate DS of about 0.5 to 1.8 using a one step aqueous process with organic acid anhydrides.
The modification of starch by chemical derivatization and in particular the preparation of starch esters is well known in the art. Techniques used in preparing the starch esters have generally involved aqueous reactions for low DS starch esters and non-aqueous systems such as anhydrous pyridine for the high DS starch esters. A good review of the preparation of starch esters may be found in R. L. Whisler, J. N. BeMiller and E. F. Paschall, Starch: Chemistry and Technology, Academic Press, 1984, Chapter X. This reference teaches that low degree of substitution (DS) starch acetates can be made by the treatment of an aqueous starch suspension with acetic anhydride at pH of 7-11. The maximum degree of substitution obtainable without gelatinization varies with the particular starch, but the upper limit is about 0.5 DS. To reach this DS of 0.5, it is necessary to repeatedly increase reagent concentrations by filtering the starch from the reaction mixture, resuspending it in 1.25-1.5 parts of water per part of starch, and continuing the acetylation.
O. B. Wurzburg, Methods in Carbohydrate Chemistry, 1964, 4, 286-287, discloses that no more than 10.2 g (0.1 mole) of acetic anhydride for 162 g (1.0) mole) of starch, dry basis, should be used when reacting an aqueous slurry of starch with acetic anhydride because the efficiency of the reaction decreases with dilution. If a higher treatment is needed, the dilution effect can be offset by removing the excess water through filtration followed by resuspension of the starch in water. The treatment can be repeated using a second portion of acetic anhydride. Treatment with 0.1 mole of acetic anhydride yields a starch acetate with a DS of about 0.07. By repeated treatment, it is possible to make a starch acetate having a DS of 0.5. For higher degrees of substitution ranging up to 3.0 DS, pyridine is the preferred catalyst in the acetylation.
Specific methods of preparing starch esters in aqueous systems are shown in the following patents. U.S. Pat. No. 2,461,139 issued Feb. 8, 1949 to C. Caldwell discloses the reaction of starch with organic acid anhydrides in an aqueous alkaline medium. Using amounts of organic acid anhydride, based on the dry starch, of 0.1 to 5% is preferred, with quantities higher than 10% being less desirable. The preparation of low DS starch esters in water using magnesium oxide or magnesium hydroxide to control pH is disclosed in U.S. Pat. No. 3,839,320 issued Oct. 1, 1974 to C. Bauer.
Accordingly, while it is known to prepare starch esters using aqueous systems as described above, such methods have been limited to the preparation of low DS derivatives and even require multiple or repeated treatments because of the difficulties that result when using higher amounts of anhydride and alkaline reagents. Such difficulties are caused by the swelling or gelatinization of the starch and the inability to filter the starch from the reaction mixture, limiting the number of repeat reactions and ultimately the maximum DS in aqueous systems. Therefore, solvent systems such as pyridine have been the preferred reaction medium for the preparation of intermediate and high DS starch esters.